1. Field of the Invention
The present invention relates to a sample stage that holds a sample, and more particularly, to a positioning technique of a sample stage apparatus including X and Y tables.
2. Description of the Related Art
Precision machines and test apparatuses use sample stages that hold samples. The sample stage requires high-speed movement and high-accuracy positioning. An example of an apparatus using the sample stage is an electron microscope.
Japanese Patent Application Laid-Open (JP-A) No. 2007-80660 (corresponding to U.S. Pat. No. 7,435,974) discloses a stage that has a driving mechanism including a stepping motor and a feed screw and performs open-loop control to achieve high-speed and high-accuracy positioning. In the driving mechanism, a gap is provided in a connection portion between a table of the stage and the feed screw in order to prevent the movement of the stage due to the thermal expansion of the feed screw. In this way, the feed screw and the table are mechanically connected to or separated from each other.
As a positioning control method of correcting a positional deviation, the following method has been proposed: a method of dividing the movement of a table into two steps and performs two processes, that is, a high-speed movement process and a low-speed movement process. The low-speed movement process stops the driving of the table at a designated position while monitoring the position of the stage using a position detector. In this way, it is possible to correct the positional deviation and achieve high-accuracy positioning.
However, in the stage positioning control disclosed in JP-A No. 2007-80660, after the high-speed movement process is performed, the table is moved by inertial force due to the gap formed in the stage connection portion, and a positional deviation occurs. The positional deviation occurring after the high-speed movement process is affected by the movement conditions of the table(for example, the inertia, the movement distance, and the movement speed of the table) or brake force. Therefore, in the case of an apparatus including a plurality of tables with different axes, the positional deviations of the tables are different from each other, and a difference in positional deviation occurs whenever the tables are moved.
That is, during the low-speed movement process performed after the high-speed movement process, a movement distance varies all the time. Therefore, the following two problems arise. (1) In the case of a stage apparatus including a plurality of tables with different axes, although the positioning of one of the tables is completed, the positioning of the other table is not completed yet, which results in a waste time for positioning. (2) A plurality of tables with different axes are not completely positioned at the same time. Therefore, when one of the tables is moved and the positioning of the other table is completed, a positional deviation occurs in the other table that has been completely positioned due to the inertia of the one table being moved.
The waste time for positioning and the positional deviation may further increase due to an increase in the speed of the table and an increase in inertia as growing the size of the table.